Eh. It feels a bit weird to me. I mean, she confessed to the other girl once and was summarily turned down, but this time, with little to no additional effort, she was accepted and it turned out the other girl always loved her. I could see that kind of stupidity happening, but it should take a bit more than a second try to overcome it.
It took a year of thinking and processing to overcome it, if that wasn't clear. Right from the get-go, probably as soon as Nanaho kissed Ayusa, Ayusa was head-over-heels for Nanaho. But she didn't want to 'drag' Nanaho into a same-sex relationship (because the world is still ignorant about romance and people are still being taught that you can only ever prescribe to one preference your entire life).
A key detail to keep in mind, Ayusa was bawling and distraught after she broke up with her girlfriend. She was completely broken down. She's not the type to cut off any serious relationship quickly and easily-if she was serious she'll break down. So none of her other relationships were real (feel bad for them), because no matter how much you try to squelch feelings for someone, if you never properly express them you'll end up holding out for them if you spend all your time with him or her.